1. Field of the Invention
The present invention relates to a gallium-nitride (GaN)-based semiconductor device and a method for manufacturing the same, and more particularly, to a layer system of the semiconductor device.
2. Related Art
Application of a GaN-based compound semiconductor to a short-wave LED (light-emitting diode) and a short-wave LD (laser diode) has been considered. In order to manufacture a GaN device, a GaN crystal must be grown on a crystal substrate, such as sapphire, which differs in lattice constant from GaN. In a known method, a buffer layer made of GaN, AlGaN, or AlN is formed to a thickness of about 20 to 30 nm at a temperature as low as about 500° C. and subsequently a GaN layer is grown at a temperature of about 1050° C. by increasing the temperature.
A GaN layer can be grown on a substrate through use of a low-temperature buffer layer. However, before growth of a low-temperature buffer layer, a substrate is usually subjected to heat treatment at a high temperature of 1100° C. or more. Therefore, a growth process becomes complicated; namely, the growth process involves the steps of heating a substrate to 1100° C., lowering the temperature; growing a buffer layer at 500° C., raising the temperature, and growing a GaN layer at 1050° C. Particularly, a step of lowering the temperature from 1100° C. to 500° C. requires considerable time, and hence manufacture of a GaN-based compound semiconductor device involves consumption of much time.
If a dislocation density of a light-emitting layer (i.e., an active layer) is high, aluminous efficiency of an LED is deteriorated, or a threshold oscillation current of an LD becomes high and the LD is deteriorated within a short period of time. For this reason, an epitaxial lateral overgrowth (ELO) method using a growth inhibition layer formed from SiO2 is also known. Under the ELO method, a GaN layer is grown on a substrate such as sapphire by means of an MOCVD (metal-organic CVD) method. After the substrate has been removed from an MOCVD apparatus, a growth inhibition layer, such as SiO2 or the like, is formed into a stripe pattern on the GaN layer by means of photolithography. Subsequently, the substrate is re-loaded into the MOCVD apparatus, and the GaN layer is again grown on the stripe-shaped growth inhibition layer. A GaN crystal starts growing on areas where no growth inhibition layer is formed and grows in both a thicknesswise direction and a lateral direction. No dislocations are transmitted to the GaN crystal that has laterally grown on the growth inhibition layer, and hence the dislocation density of the GaN crystal is restrained.
However, even under the ELO method, the growth inhibition layer must be formed through use of photolithography, thus requiring efforts. Moreover, the substrate must be removed from the MOCVD apparatus for forming the growth inhibition layer and then loaded into the MOCVD apparatus, and hence the surface of the substrate is vulnerable to contamination.